Traditional hybrid fibre-coax (HFC) architectures have been deployed with a downstream one-way-only broadcast service requirement, with no, or limited cable return path. Recently, as network providers and equipment manufacturers have started to explore the options for high capacity two-way services, the limitations of the original system choices have become apparent. A symmetrical bandwidth or a more symmetric bandwidth than at present is described. This will allow the development of high capacity 2-way services. Examples of such two way (more symmetric) services include video telephony and work-at-home data applications (as opposed to INTERNET browsing). Such 2-way or highly interactive services enable more economic growth of HFC networks in the future.
Data services used for downloading from central servers are asymmetric, but can be highly symmetric if they are used for work-at-home or remote "grass roots" publishing applications. In the latter case, each subscriber's "download" becomes the publisher's "upload". LAN extension applications, assuming Ethernet 10BaseT connections within the home requires instantaneous 2-way bandwidths of up to 10 Mb/s, if the HFC plant is not to be the bottleneck although the net throughput would be considerably less than this. Preferably latency must be kept low, nominally less than 10 msec, particularly in a LAN environment. As opposed to this, Internet surfing may also benefit from a large downstream capability, however in this case a few 10's of kb/s upstream is likely to be sufficient for this application.
Video conferencing bandwidths are symmetric although lower than those for data or entertainment--384 Kb/s to 2 Mb/s. Holding times however could be several hours, which impacts the upstream capacity engineering of the access plant. To the extent that any of the data or video services are used for business applications, they would have to have a reliability requirement approaching that of telephony today. The reliability requirement also affects the engineering choices in the access plant, requiring a small failure group size.
The upstream capacity is limited in hybrid fiber coaxial designs by the characteristics of the spectrum--the low end of the band in the 5 to 42 MHz region--to which upstream traffic has been assigned in cable industry practice. This spectrum has problems with a high noise floor due in part to ingress by subscriber's cable plant noise. The presence of noise limits both the absolute amount of bandwidth available for upstream use, and the maximum bandwidth of any single channel. In practice, the limit is about 5 to 8 channels of 2 MHz each, which results in about 10 to 16 Mb/s of total bandwidth in a TDMA system, factoring in modulation index and overheads. Furthermore, the modems need to be dynamically agile to be able to seek quiet areas of the spectrum as interference conditions change.
The use of the high end of the band (350 to 500 MHz) allows larger contiguous blocks of bandwidth and such bandwidth is subject to less noise sources. Higher attenuation losses are incurred due to the higher frequencies, but the reduced transmission path distances caused by placing the electro-optic conversion plant deep in the access network (i.e. closer to the subscriber) avoid the need for expensive additional bi-directional amplifiers.
A design that uses remote digital electronics (an access node) to provide high upstream bandwidth to a group of around 240 subscribers provides higher reliability than existing analogue systems and constrains the failure group size to limits normally acceptable in the industry for telephony. Average upstream bandwidth per subscriber can be further increased by further reducing the number of customers served by the access node. This can be acheived by dividing the amount of feeder coax served i.e. reducing the number of "coax legs" from any single access node. This has the effect of reducing both noise ingress and the total demand for upstream bandwidth on that access node.
To accommodate a diverse service mix such as LAN, video conferencing, games and VOD, it would be desirable to have a transport layer which utilizes the ATM format to carry all information and signaling between the central office and the home. The ATM format has been extended to the periphery of the broadband network since it provides a simple, consistent method for handling virtual circuits of different bandwidths and of different delay, burst characteristics and Quality of Service requirements over a single network. Further, ATM supports evolution of the MPEG bit rate as silicon speeds increase and more sophisticated decoding techniques are developed.
A cable system should preferably have the following key characteristics: the ability to co-exist with existing broadcast cable architectures; high capacity in both the downstream and upstream direction for switched digital services, achieved by inserting and extracting bandwidth deep in the outside plant, closer to the subscriber than traditional headend broadcast architectures; the ability to support a wide range of video and data services without preconfiguration; support of low latency upstream requirements; flexible deployment options to allow for increasing penetration of high upstream bandwidth interactive services.
The architecture of multiple users sharing bi-directional bandwidth on a shared access medium leads to a number of implementation possibilities: firstly it is assumed that there is little community-of-interest between adjacent subscribers and any such traffic can be "hairpinned" via the core network. This allows simplification of the system to a point to multi-point (rather than a multi-point to multi-point). In the downstream direction (point to multi-point) traffic is "broadcast" and is received at all subscribers. A single downstream FDM channel could be allocated (either statically or dynamically) to each subscriber within the coax leg to limit access, but this requires a large number of different frequency modems at the access node and either frequency adaptable modems or a large number of variant outstation types. It also complicates configuration of the network and obviously limits the peak downstream bandwidth to any one subscriber. Thus a TDM method is preferable to allow bursty downstream bandwidth. This could either be a single very broad TDM channel or a small numbers of less broad TDM channels, each one allocated statically to a block of subscribers--which implementation is adopted makes no difference to this invention.
In the upstream direction (multi-point to point), various possibilities can be considered, amongst them: FDMA, TDMA and CDMA. FDMA on its own is ruled out for the same reason as FDM . CDMA allows all users access to the whole bandwidth, but the cost/complexity of CDMA where the subscribers/outstations are capable of continually varying data rates outweighs the benefits. Due to the uniform ATM PDU size, a TDMA system based on ATM presents a potential solution where each TDMA timeslot holds a single ATM cell and use of such timeslots could be individually allocated in a dynamic fashion. This allows simple sharing of bandwidth between a number of users, thus allowing concentration of CBR type connections (e.g. voice calls) and statistical gain of bursty connections. However, to limit the bandwidth required of a single upstream modem and to allow work-around known interferers, it is proposed to support a limited number of TDMA channels operating at separate frequencies. An individual subscriber would be statically or semi-statically assigned to a TDMA channel. While this system could be described as FDMA/TDMA, the present invention does not differentiate between subscribers operating at differing frequencies and therefore TDM can be used downstream and TDMA can be used upstream.
As is known, TDMA systems often have marshalling "slots"--gaps in upstream traffic which are used to accommodate the unknown round trip delay when outstations first "sign on". The present invention relates to a system whereby the basestation commands an outstation (or group of outstations) to transmit a known marshalling sequence inside a known marshalling window. Only unmarshalled outstations need respond. Outstations only respond when commanded to by the basestation. The existence of any signal is used to detect unmarshalled outstations. Once a unique unmarshalled outstation has been resolved, then the phase and amplitude of the received known marshalling sequence is used to adjust the launch phase and amplitude of the outstation. The marshalling window needs to be long enough to accommodate variations in round trip delay plus the length of the marshalling sequence chosen to ensure accurate phase/amplitude measurements.
For systems with an appreciable round-trip delay, such as cable networks, use of a dedicated marshalling window can amount to a significant usage of the TDMA bandwidth.
One solution that is to increase the "frame length" i.e. increase the period over which there is a regular marshalling slot. However, this leads to slower marshalling and increased latency of other functions which rely on a small frame length.
It is an object of this invention is provide a TDMA protocol operable in a cable network wherein the marshalling slots do not take up appreciable bandwidth.